The countercurrent extraction of roast ground coffee is well-known in the art and has been accepted as one of the most practical means for producing beverage products. Countercurrent extraction involves a process in which the aqueous medium utilized to extract the coffee first contacts the coffee which has been on-stream the longest period of time. As the medium passes through the extraction zone, it successively contacts coffee having higher and higher contents of coffee solubles. Just prior to exit from the zone, it passes through the freshest of the coffee which is on-stream.
Countercurrent extraction is conventionally practiced through the use of an extraction zone having a plurality of individual percolator or extraction cells, ordinarily three or more. These cells or containers each hold roast ground coffee extracted to greater or lesser degrees and are serially connected so that the aqueous extraction medium passes consecutively through each.
Extraction consists of a series of continuous cycles. Thus, after a pre-selected on-stream period--the time being dependent upon the weight of the extraction medium--i.e. "draw-off"--desired passed through each cell, the cell sequence is shifted. The selection of an appropriate time period, and the corresponding weight of draw-off, depends upon the desired degree of extraction of the coffee. This selection is in turn reflected by the draw-off ratio, which is the weight of aqueous extract to the weight of coffee in a cell. This ratio determines the yield and quality of the eventual beverage product.
The shift is performed through the essentially simultaneous removal of the cell containing the most exhausted or extracted coffee--i.e. the cell at the end of the zone at which the aqueous extraction medium enters--and the addition of a new cell containing fresh, unextracted roast ground coffee at the other end of the zone. Each such shift marks the end of one cycle and the beginning of the next.
A drawback of this prior art countercurrent extraction process lies in the fact that, despite the continuity of extraction cycles, the production of extract is discontinuously cyclic. The new cell added at the downstream end of the extraction zone is initially filled only with particulate roast ground coffee. Thus no extract exits from the zone until that cell first fills with extraction medium. Accordingly, there is a time lag in outflow of extract at the beginning of each cycle.
Also in accordance with prior art coffee beverage processing techniques, extract exiting the extraction zone is customarily first cooled in order to protect against thermally induced flavor degradation and aroma loss and then measured to monitor the yield and ensure the most efficient draw-off for each cycle. Once measured, as by weighing, it is ordinarily passed to further processing which, for example, includes stripping of coffee extract volatiles temporarily to isolate and preserve these materials, concentration of stripped extract, reconstitution of extract with isolated volatiles and then drying.
Because stripping of volatiles is performed at high temperature and, most efficiently, on a continuous basis, further intermediate steps often occur between measuring and stripping the extract. Thus to avoid sporatic stripping resultant from the fact that the extract is not continuously obtained from the extraction zone, the extract is normally introduced into a holding tank. There a sufficient quantity of extract is maintained to permit continuous and steady stripping. Additionally, because the extract has ordinarily been cooled immediately after exit from the extraction zone, it is normally reheated immediately prior to, or during, stripping.